1. Field of the Invention
The present invention relates to current mirrors and, more particularly, but not by way of limitation, to a low noise apparatus for producing an output current which mirrors the input current.
2. Description of the Related Art
Audio chips presently enable personal computers, compact disk players, and other portable audio devices to execute high quality, low power audio applications. Audio chips usually comprise digital circuitry which occupies approximately 75-80% of the audio chip's silicon space and analog circuitry which occupies the remaining 20-25%. Typically, the analog circuitry comprises an analog-to-digital converter, a digital-to-analog converter, and some output amplifiers. The analog circuitry converts an analog audio input signal into a digital format suitable for processing by the digital circuitry. Also, the analog circuitry converts the digital signals back into an analog format suitable to drive a load, such as a speaker. The digital circuitry occupies the majority of the silicon area and typically performs digital signal processing, such as filtering, noise shaping, and synthesizing on the converted analog signals. The primary function of these audio chips is to implement an entire audio system on one piece of silicon.
The above-described analog circuitry typically comprises current mirrors. These current mirrors serve several important functions, such as providing reference currents and reference voltages to other components in the analog circuitry. Therefore, these current mirrors must have very good matching characteristics and low noise (i.e., must have a large signal to noise ratio) to improve, illustratively, the output swing of the output amplifiers and the overall reliability and accuracy of the analog circuitry.
FIG. 1 illustrates current mirror 100, which is a conventional cascode current mirror comprising N-channel transistors 110, 120, 130, and 140. Transistors 110, 120, 130, and 140 are enhancement-type, metal-oxide silicon field effect transistors (i.e., MOSFETs). For the output current (i.e., I.sub.OUT) Of current mirror 100 to exactly match (i.e., mirror) the input current (i.e., I.sub.IN), transistors 110 and 130 must have identical threshold voltage drops (i.e., V.sub.T) and gate-to-source voltage drops (i.e., V.sub.GS). Similarly, transistors 120 and 140 must have identical threshold voltage drops (i.e., V.sub.T) and gate-to-source voltage drops (i.e., V.sub.GS). These requirements for current mirror 100 will become evident from the equations defining I.sub.OUT and I.sub.IN (described herein).
Transistors 120 and 140 have identical V.sub.GS because their sources are connected to a reference voltage (e.g., ground) and their gates are connected to each other. Similarly, transistors 110 and 130 have nearly identical V.sub.GS because their gates are connected to each other and they have identical drain currents.
Moreover, to have identical V.sub.GS and V.sub.T drops, transistors 110 and 130 must be equal in size (i.e., width and length) and transistors 120 and 140 must be equal in size. Therefore, transistors 110 and 130 and transistors 120 and 140 are fabricated to be as close in size as possible. Unfortunately, however, two exactly sized transistors cannot be fabricated due to inherent errors associated with currently available fabrication techniques. Consequently, the V.sub.T of transistors 120 and 140 and transistors 110 and 130 are not identical. A first-order model of this threshold voltage mismatch (i.e., .DELTA.V.sub.T) between transistors 120 and 140 is illustrated in FIG. 2.
Referring to FIG. 2, the input current I.sub.IN of current mirror 100 can be approximated by the following equation: EQU I.sub.IN =(k')(w/l)(V.sub.GS -V.sub.T).sup.2 (1)
where k' is a process parameter, w/l is the size (i.e., width and length) of transistor 120, V.sub.T is the threshold voltage of transistor 120, and V.sub.GS is the gate-to-source voltage of transistor 120.
The voltage at the gates of transistors 120 and 140 (i.e., V.sub.A) can be approximated by the following equation: EQU V.sub.A =.DELTA.V.sub.T +V.sub.GS (2)
Therefore, substituting equation (2) into equation (1) and solving for V.sub.A : EQU I.sub.IN =(k')(w/l)[V.sub.A -.DELTA.V.sub.T -V.sub.T ].sup.2 EQU V.sub.A =.DELTA.V.sub.T +V.sub.T +[I.sub.IN /(k'(w/l))].sup.1/2 (3)
Similarly, I.sub.OUT may be approximated by the following equation: EQU I.sub.OUT =(K')(w/l)(V.sub.GS -V.sub.T).sup.2 (4)
where k' is the process parameter, w/l is the size (i.e., width and length) of transistor 140, V.sub.T is the threshold voltage of transistor 140, and V.sub.GS is the gate-to-source voltage of transistor 140. Substituting equation (2) into equation (4) and solving: EQU I.sub.OUT =(k')(w/l)[V.sub.A -V.sub.T ].sup.2 (5)
Substituting equation (3) into equation (5) and solving: ##EQU1## Accordingly, the first order and second order terms 2(k')(w/l)(.DELTA.V.sub.T)[I.sub.IN /(k'(w/l))].sup.1/2 and k'(w/l)(.DELTA.V.sub.T).sup.2 (see equation 6) are error terms resulting from the threshold voltage mismatch .DELTA.V.sub.T.
Illustratively, if I.sub.IN =50 .mu.A, k'=43.times.10.sup.-6 A/V.sup.2, w/l=100/10, and .DELTA.V.sub.T =10 mV, then: EQU I.sub.OUT =50.times.10.sup.-6 +2.61.times.10.sup.-6 +0.034.times.10.sup.-6 EQU I.sub.OUT =52.644 .mu.A
Thus, for an input current of 50 .mu.A, the output current of current mirror 100 is 52.644 .mu.A. This disparity in input and output currents produces an error rate of 5.3% The majority of this error is attributable to the first order error term in equation 6. Therefore, if a new and improved current mirroring apparatus could be designed which would significantly reduce the mismatch/noise and, thus, the error rate resulting from the threshold voltage mismatch .DELTA.V.sub.T, the overall reliability and accuracy of the analog circuitry would be greatly increased.